I’m building a bookshelf out of MDF (18mm thick) using 6mm dowels (Silverline brand like the dowelling jig). Each shelf will be dowelled in place at the back and on both sides.

I’m concerned about whether the dowels will be strong enough for a heavily loaded shelf.

Is there a formula or rule of thumb that says how many 6mm dowels are required (or even what distance between dowels is required) to support a given shelf load (in kg) assuming the shelf is supported only by dowels on 3 sides?

I’m planning for the dowels to be glued. Are there times when dowels are not enough and the shelf should also be glued, screwed or nailed in place?

There's no formula that I know of, and if there were it would need to be taken with a pinch of salt because dowel strength can vary enormously due to variation from piece to piece (see this recent thread for a little on why). Additionally the length of the dowels and whether they go through the case or are in shallow holes are important factors, as is the quality of the MDF (it's not all equal, and the worst of it is pretty crumbly).
– GraphusJun 26 at 13:12

I think because of these many reasons, a lot of people switch to biscuits for MDF work like this.
– jdvJun 26 at 13:15

2

One last pithy comment is that whenever I think about MDF work I always consider flat-pack furniture construction (the good and the bad) we've all seen too much of. How do Ikea, et al, engineer this sort of thing? What construction do they use for their items intended for more massive static forces? This will give you an idea of how they tune their construction for the material at hand and the loads involved.
– jdvJun 26 at 13:23

3

@jdv "I think because of these many reasons, a lot of people switch to biscuits for MDF work like this." tee hee. I'm laughing because testing shows that dowel joinery is, and this is a surprise to most, actually hard to beat in terms of strength. I believe what happens when pros are weighing whether to go with a biscuit jointer or dowels that speed becomes the sole deciding factor in the majority of cases, despite the much higher initial spend and the higher ongoing costs with biscuits. Which is perfectly legit of course, clearly both are strong enough for most purposes, and time is money.
– GraphusJun 26 at 16:57

1

Metal would certainly be stronger.... but it's possibly overkill for a shelving unit where the shelf material itself may be likely to turn out to be the weak point ^_^ That aside, this doesn't Answer what was asked. It's fine to suggest alternatives, but it's considered good form to answer the Question as asked, then go on to give the alternate suggestion(s) if you have some and ideally with explanations for why they're better/more desirable, (e.g. stronger, less work, cheaper) or necessary (only safe to do it by a different method than asked).
– GraphusJun 30 at 5:05